The present invention relates to apparatus for, and the processing of, semiconductor wafers. In particular, the invention relates to the deposition of antireflective layers during wafer processing.
In the manufacture of integrated circuits, photolithographic techniques are used to define patterns for layers in an integrated circuit. Typically, such photolithographic techniques employ photoresist or other light-sensitive material. In conventional processing, the photoresist is first deposited on a wafer, and then a mask having transparent and opaque regions which embody the desired pattern, is positioned over the photoresist. When the mask is exposed to light, the transparent portions allow light to expose the photoresist in those regions, but not in the regions where the mask is opaque. The light causes a chemical reaction to occur in the exposed portions of photoresist. A suitable chemical, or a chemical vapor or ion bombardment process, then is used to selectively attack either the reacted or unreacted portions of the photoresist. With the photoresist pattern remaining on the wafer itself now acting as a mask for further processing, the integrated circuit can be subjected to additional process steps. For example, material may be deposited on the circuit, the circuit may be etched, or other known processes carried out.
In the processing of integrated circuit devices with small feature sizes, for example, feature sizes having critical dimensions less than one-half micron, sophisticated techniques involving equipment known as steppers, are used to mask and expose the photoresist. The steppers for such small geometry products generally use monochromatic (single-wavelength) light, which enables them to produce very fine patterns. As repeated process steps are carried out, however, the topology of the upper surface of the substrate becomes progressively less planar. This uneven topology can cause reflection and refraction of the monochromatic light, resulting in exposure of some of the photoresist beneath the opaque portions of the mask. As a result, this differing local substrate surface typography can alter the fine patterns of photoresist, thereby changing the desired dimensions of the resulting regions of the semiconductor substrate.
In the manufacture of semiconductor devices, it is desirable that fluctuations in line width, or other critical dimensions, be minimized. Errors in such dimensions can result in open or short circuits, thereby ruining the resulting semiconductor devices. As a result, some semiconductor manufacturers now require that the dimensional accuracy of a photoresist pattern be within 5 percent. To achieve that dimensional accuracy, two approaches have been taken. Both approaches entail the use of another layer in addition to the photoresist layer.
The first approach uses a relatively thick organic film beneath the photoresist that absorbs incident light so that minimal reflection or refraction occurs. A disadvantage of such organic films is that they require more process steps, and being polymer-based, are difficult to etch.
A second approach is the use of an antireflective film for canceling reflections occurring at the photoresist-antireflective layer interface, and at the antireflective layer-substrate interface. In the prior art, silicon oxynitride (SiON) deposited using NH3 gas has been used as an antireflective film. Upon exposure to light, however, an amino group from the SiON film reacts with the light sensitive component in the photoresist, thereby desensitizing the photoresist. This results in inaccurate photoresist patterns.
An article entitled “SiOxNy:H, high performance antireflective layer for the current and future optical lithography,” SPIE, Vol. 2197 (1994), pp. 722–732, by Tohro Ogawa, et al., addresses the thin film interference concerns. The article teaches the use of an antireflective layer (ARL) in conjunction with the I-line, KrF, and ArF excimer laser lithographies. The exposure wavelengths used in these laser lithographies are 365 nm, 248 nm, and 193 nm, respectively. The article describes that as exposure wavelengths become shorter, stronger reflections from the interface between the photoresist and the substrate result. Hence, an ARL is needed to reduce the standing waves and thin film interference effects.
This ARL is described as canceling reflection from both the interface between the photoresist and the ARL, and from the interface between the ARL and the substrate. The article describes a complicated equi-energy contour-based procedure for determining the parameters to achieve the desired cancellation. According to the procedure described in this article by Sony, the parameters are obtained by finding common regions of the equi-energy contour lines for a plurality of photoresist film thicknesses. The article describes refractive index, absorptive index, and thickness values for its ARL, and though the article does not specify it, Applied Materials inventors have determined that these values correspond to a phase shift of 180° between the reflections. The Applied engineers, however, were unable to achieve the results described in the article, and it is believed that the process is unstable.
Sony has also filed a European patent application (Application No. 93113219.5, Publication No. EP 0 588 087 A2) for a process for depositing an ARL with selected parameters. The Sony application discusses the SiH4 and N2O ratio, and how the ratio affects the optical and chemical properties of the ARL deposited. The Sony application also teaches the use of argon.